The present invention is generally directed to the field of boring tools. The present invention has particular applicability to the field of boring heads of the type used in vertical lathes to produce large diameter bores. The invention is especially useful in producing the bore in the stator yoke of a large electric motor, e.g. a 10,000 hp AC induction motor.
An exemplary stator yoke 10 is shown in FIGS. 1A, 1B, 1C and 1D, as used in the 1100 Frame 10,000 hp AC motor produced by Siemens Energy and Automation, Inc. of Norwood, Ohio. Such a yoke 10 is about 89.8 inches in length and includes a yoke bore 12 of about 38 inches in diameter, machined along an axis extending the length of the yoke 10. Further, the yoke 10 is fashioned with a number of structural ribs 14, preferably six, that support the stator and provide void areas therebetween for cooling ventilation and weight reduction of the assembled motor. In machining, the yoke bore 12 is ground from the surfaces of the ribs 14. Three-eighths (xe2x85x9c) to one-half (xc2xd) inch of material must be removed and the bore 12 must be held to a tolerance of about 0.001. This presents a problem during the boring process since the ribs 14 essentially define a non-continuous xe2x80x9cinterruptedxe2x80x9d surface to be machined.
FIG. 2 illustrates a typical previous-type vertical lathe 20 used in a typical boring operation for a large diameter yoke bore 12. A torque is applied to the lathe 20 which transmits a load to a cutting tool 22 of carbide steel, in order to grind the surfaces of the ribs 14. However, when the cutting tool 22 comes away from the interrupted surface of the ribs 14, the load is abruptly released. As the cutting tool 22 comes in contact with the next rib 14, the load is abruptly reapplied. This interrupted operation results in variable torque and resulting wear and tear upon the lathe motor. Also, the load must be limited to avoid overheating the cutting tool 22, which would reduce tool life and necessitate frequent tool changes, with increased expense and reduced production efficiency. Consequently, the lathe spindle is fed into the work at about 0.0075 inches per revolution, with a spindle rate of about 60 revolutions per minute (RPM), in order to obtain a permissible load on the lathe motor and to minimize wear and tool changes for the cutting tool 22. At this rate, it takes about three hours to cut the yoke bore 12. Thus, the boring process is quite time-consuming, contributing significantly to the production time and thus expense of the product.
In view of the difficulties and drawbacks associated with previous machining tools and processes, there is therefore a need for a machining tool and process that decreases bore time.
There is also a need for a machining tool and process that decreases tool and machine wear.
There is also a need for a machining tool and process that increases production efficiency.
These needs and others are satisfied by the boring head of the present invention in which a plurality of boring tools provide a plurality of simultaneous cuts to a workpiece. A tool holder supports the plurality of boring tools generally perpendicular to a bore axis, so as to increase the rate of a boring operation.
As will be realized, the invention is capable of other and different embodiments and its several details are capable of modifications in various respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative and not restrictive.